The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to an image transfer device included therein and a charge roller, developing roller, transfer roller, cleaning roller or similar roller joining in image formation.
Generally, an image forming apparatus of the kind described includes a photoconductive drum or similar image carrier. An image transfer device includes a transfer roller pressed against the lower portion of the image carrier with opposite end portions thereof pressed by a pressing device. A charger uniformly charges the surface of the image carrier with a charge roller. A developing unit includes a developing roller for developing a latent image electrostatically formed on the image carrier. A cleaning unit includes a cleaning roller for removing toner left on the image carrier after image transfer. A bias power source applies a bias for image transfer to the transfer roller, so that the transfer roller transfers an image from the image carrier to a paper or similar recording medium.
Japanese Patent Laid-Open Publication No. 52-80842, for example, teaches an electrophotographic copier including a photoconductive drum and an image transfer device including a transfer charger. The transfer charger adjoins the photoconductive drum. A paper guide implemented as a guide roller is positioned upstream of the transfer charger in a direction of paper transport in the vicinity of the photoconductive drum. The guide roller has a greater outside diameter at its center than at its opposite ends, so that the center portion of a paper reached the roller first contacts the drum.
Japanese Patent Laid-Open Publication No. 4-321082 discloses an image forming apparatus including a photoconductive drum and an image transfer device implemented as an elastic transfer roller pressed against the drum. The transfer roller transfers an image from the photoconductive drum to a paper being conveyed between the transfer roller and the drum. The transfer roller has a circumferential surface configured in an inverse crown.
Japanese Patent Laid-Open Publication No. 4-127176 proposes an image forming apparatus including a rotatable charge roller for depositing a preselected potential on a photoconductive element. A developing roller develops a latent image formed on the photoconductive element by exposure with a developer. A transfer roller transfers the resulting toner image from the photoconductive drum to a paper. A cleaning roller removes toner left on the photoconductive element after the image transfer. At least one of the above rollers is uniformly pressed against the photoconductive element in its axial direction by leaf springs.
Japanese Patent Laid-Open Publication No. 2-24685 discloses an image forming apparatus including a movable image carrier and a conductive, elastic transfer roller adjoining the image carrier and movable in synchronism with the image carrier. A paper is fed to a nip between the image carrier and the transfer roller. The transfer roller has a greater outside diameter at its center portion than at its opposite end portions.
Further, Japanese Patent Laid-Open Publication No. 8-234545 proposes an image forming apparatus including a transfer roller positioned above a photoconductive drum.
Transfer rollers in general have a metallic core and a sag problem, as follows. While a transfer roller with a metallic core having a great diameter does not sag, it is bulky and heavy. If the diameter of the metallic core is small, then the transfer roller sags due to its own weight. As the transfer roller is made longer, it sags more and cannot maintain a uniform nip width between it and an image carrier.
More specifically, so long as the transfer roller is shorter than 350 mm, the transfer roller does not sag by more than 0.1 mm even when its core (formed of iron) has a relatively small diameter (8 mm). However, the sag sharply increases when the above length exceeds 350 mm. Because the transfer roller is usually pressed against the lower portion of an image carrier, the nip width is smaller at the center portion than at the opposite end portions due to the sag of the roller. This is likely to bring about defective images that are locally lost. On other hand, assume that the transfer roller is positioned above an image carrier and pressed against the image carrier with its opposite ends pressed by a pressing device. Then, the nip width is greater at the center portion than at the opposite end portions due to the sag of the transfer roller, resulting in irregular image transfer and therefore defective images.
The above sag problem also occurs with a charge roller, a developing roller, a cleaning roller and other rollers arranged in an image forming apparatus. For example, the charge roller, developing roller and cleaning roller respectively render charging, development and cleaning irregular due to their sag, also resulting in defective images.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-186812 and 7-225523.
It is therefore an object of the present invention to provide an image forming apparatus capable of obviating defective images ascribable to the sag of rollers included therein.
An image forming apparatus of the present invention includes an image carrier and an elastic transfer roller pressed against the image carrier and conveys a recording medium between the transfer roller and the image carrier to thereby transfer a toner image from the image carrier to the recording medium. The transfer roller is positioned in an angular range of less than xc2x190xc2x0 from the top of the image carrier in the direction of rotation of the image carrier. The transfer roller has a greater diameter at its opposite end portions than at its center portion in the axial direction of the roller. Alternatively, the transfer roller may have lower hardness, lower density or lower electric resistance at the opposite end portions than at the center portion. This roller configuration is similarly applicable to any other roller included in the image forming apparatus.